Automatic test equipment has been used for many years to evaluate the performance of ICs prior to shipment to customers. With the steadily increasing complexity of ICs, there has been a corresponding need for higher speed test equipment, so that the overall time for testing can be kept within reasonable limits. Recently, ICs have become so complex (e.g. with 500 or so pins per chip) that the test equipment must be able to operate at a toggle rate as high as 200 MHz. At that rate, each test cycle must be completed within the 5 nano-second (5.times.10.sup.-9 seconds) cycle time of the test sequencing.
Such automatic test equipment includes a large number of "pin cards" carrying circuitry for communicating with a corresponding pin of the device under test. Each pin card includes a so-called "pin driver" which is connectable to a corresponding pin of the device under test to supply a test signal thereto. The output voltages at one or more other pins of the device are monitored during the test period, and are compared to programmed voltages to determine whether applicable specifications have been met. During the test period, the pin being monitored also must be connected to an active load which forces a predetermined current into (or out of) the pin, to insure that proper test conditions are maintained at the time of testing. During each test period, each pin will be connected to either a pin driver or to an active load, depending upon whether the pin is to receive a test signal, or whether its output response is to be measured.
As the test program proceeds, many of the pins must be sequentially switched between the pin driver and the active load of the corresponding pin card. Thus, the pin driver and the active load must be capable of being connected and disconnected from the corresponding pin at very high speed so as to achieve proper test conditions within each test period. Most importantly, when the active load is being turned off (in response to an "inhibit" signal), extremely fast switching for the turn-off is essential in order to be sure that the current into or out of the active load does not affect the pin driver which is being turned on at about the same time.